1. Field of the Invention
The field of this invention relates to sampling and downhole testing techniques for subterranean formation cores, particularly applications using continuous nuclear magnetic resonance analyses of formation cores in a measurement-while-drilling mode.
2. State of the Art
It is desirable for the well operator to test the properties of the formation adjacent the wellbore. Frequently, properties such as permeability and porosity are measured using techniques, including, but not limited to, nuclear magnetic resonance (NMR), X-ray, or ultrasonic imaging.
One way of using techniques for measurement of formation properties is to drill the hole to a predetermined depth, remove the drillstring, and insert the source and receivers in a separate trip in the hole and use NMR to obtain the requisite information regarding the formation. This technique involves sending out signals and capturing echoes as the signals are reflected from the formation. This technique involved a great deal of uncertainty as to the accuracy of the readings obtained, in that it was dependent on a variety of variables, not all of which could be controlled with precision downhole.
Coring has also been another technique used to determine formation properties. In one prior technique, a core is obtained in the wellbore and brought to the surface where it is subjected to a variety of tests. This technique also created concerns regarding alteration of the properties of the core involved in the handling of the core to take it and bring it to the surface prior to taking measurements. Of paramount concern was how the physical shocks delivered to the core would affect its ability to mimic true downhole conditions and, therefore, lead to erroneous results when tested at the surface.
Other techniques have attempted to take a core while drilling a hole and take measurements of the core as it is being captured. These techniques which have involved NMR are illustrated in U.S. Pat. Nos. 2,973,471 and 2,912,641. In both of these patents, an old-style bit has a core barrel in the middle, which rotates with the bit. As the core advances in the core barrel as a net result of forward progress of the bit, the core passes through the alternating current and direct current fields and is ultimately ejected into the annulus.
The techniques shown in the two described patents have not been commercially employed in the field. One of the problems with the techniques illustrated in these two patents is that the core integrity is destroyed due to the employment of a rotating core barrel. The rotating core barrel, which moves in tandem with the bit, breaks the core as it enters the core barrel and before it crosses the direct current and radio frequency fields used in NMR. The result was that unreliable data is gathered about the core, particularly as to the properties of permeability and porosity which are greatly affected by cracking of the core. Additionally, the physical cracking of the core also affected readings for bound water, which is water that is not separable from the core mass.
An apparatus is disclosed that allows the taking of cores during drilling into a nonrotating core barrel. NMR measurements and tests are conducted on the core in the nonrotating barrel and, thereafter, the core is broken and ejected from the barrel into the wellbore annulus around the tool. In conjunction with a nonrotating core barrel, a sub is included in the bottomhole assembly, preferably adjacent to the bit, which, in conjunction with an inclinometer of known design, allows for real-time ability to control the movement of the bit to maintain a requisite orientation in a given drilling program. The preferred embodiment involves the use of a segmented permanent magnet to create direct current field lines, which configuration facilitates the flow of drilling fluid within the tool around the outside of the core barrel down to the drill bit so that effective drilling can take place.
The apparatus of the present invention overcomes the sampling drawbacks of prior techniques by allowing a sample to be captured using the nonrotating core barrel and run past the NMR equipment. Various techniques are then disclosed to break the core after the readings have been taken so that it can be easily and efficiently ejected into the annular space. A steering mechanism is also provided, as close as practicable, to the drill bit to allow for orientation changes during the drilling process in order to facilitate corrections to the direction of drilling and to provide such corrections as closely as possible on a real-time basis while the bit advances. The specific technique illustrated is usable in combination with the disclosed nonrotating core barrel, which, due to the space occupied by the core barrel, does not leave much space on the outside of the core barrel to provide the necessary mechanisms conventionally used for steering or centralizing.
Another advantage of the present invention is the provision of components of the NMR measurement system in such a configuration as to minimize any substantial impediment to the circulating mud which flows externally to the core barrel and through the drill bit to facilitate the drilling operation.